realtek: resurrect timer driver

realtek: resurrect timer driver

Now that we provide a clock driver for the Reltek SOCs the CPU frequency might
change on demand. This has direct visible effects during operation

- the CEVT 4K timer is no longer a stable clocksource
- after CPU frequencies changes time calculation works wrong
- sched_clock falls back to kernel default interval (100 Hz)
- timestamps in dmesg have only 2 digits left

[    0.000000] sched_clock: 32 bits at 100 Hz, resolution 10000000ns, wraps ...
[    0.060000] pid_max: default: 32768 minimum: 301
[    0.070000] Mount-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
[    0.070000] Mountpoint-cache hash table entries: 1024 (order: 0, 4096 bytes, linear)
[    0.080000] dyndbg: Ignore empty _ddebug table in a CONFIG_DYNAMIC_DEBUG_CORE build
[    0.090000] clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, ...

Looking around where we can start the CEVT timer for RTL930X is a good basis.
Initially it was developed as a clocksource driver for the broken timer in that
specific SOC series. Afterwards it was shifted around to the CEVT location,
got SMP enablement and lost its clocksource feature. So we at least have
something to copy from. As the timers on these devices are well understood
the implementation follows this way:

- leave the RTL930X implementation as is
- provide a new driver for RTL83XX devices only
- swap RTL930X driver at a later time

Like the clock driver this patch contains a self contained module that is SOC
independet and already provides full support for the RTL838X, RTL839X and
RTL930X devices. Some of the new (or reestablished) features are:

- simplified initialization routines
- SMP setup with CPU hotplug framework
- derived from LXB clock speed
- supplied clocksource
- dedicated register functions for better readability
- documentation about some caveats

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
[remove unused header includes, remove old CONFIG_MIPS dependency, add
REALTEK_ prefix to driver symbol]
Signed-off-by: Sander Vanheule <sander@svanheule.net>